This invention relates to a method of producing a contact device for a switch, especially a vacuum switch, in which the contact alloy is formed by powder sintering.
According to the method of powder sintering, several kinds of metal powders or intermetallic compounds are used as a starting material and are thoroughly mixed by mechanical means. The resulting mixture is pressed at a predetermined molding pressure and sintered within a high-temperature atmosphere to become a sintered body for use as a contact alloy material. The sintered body thus obtained is machined to have a desired shape and brazed to a substrate made of copper or the like electrically conductive material thus completing the contact device.
For attaching the contact to the substrate, soldering is usually resorted to by using a hard solder of the Cu-Ag type or Cu-Au type under vacuum or in a hydrogen atmosphere. However, since the hard solder does not fuse well with the contact alloy, the conventional practice is by etching the bonding surface of the contact or by depositing a more adhesive coating layer on the bonding surface of the contact through vacuum metallization.
In order to avoid deterioration of the contact performance due to contamination of the operating contact surface with the coating layer or the ethching liquid, the surface of the contact need be masked except for the surface to be bonded to the substrate by hard soldering. However, in spite of the additional time and labor involved in the masking operation, it has not been possible to realize a sufficient bonding strength between the contact and the substrate. On the other hand, when the contents of low melting metals, such as Bi, in an alloy are increased to be higher than 0.5 percent with the object of improving the welding resistance of the contact and reducing the chopping current produced by the on/off contact operation, the solderability of the alloy is necessarily reduced.
A device shown in FIG. 1 has been used in the prior art for holding the contact in position. In the figure, a contact 1 formed with a lower flange 2 is placed on a substrate 3 and an annular mounting member 4 is fitted and brazed to the substrate 3 for securing the contact 1 thereto. Numeral 5 designates an electrode brazed to the substrate, and numeral 6 designates a brazing layer or a layer of hard solder.
In the construction shown in FIG. 1, the contact 1 can be held in place when a material that fuses well with hard solder is used for the substrate 3 and the lower flange 2. However, since the bonding between the contact 1 and the substrate 3 is not improved in this case, the holding device tends to be complicated in structure.
FIG. 2 shows another system for holding the contact in position. In the figure, a groove 7 is formed on sides of the contact 1, and the end of a mounting member 8 is bent and caulked in the groove 7 for holding the contact 1. The mounting member 8 is brazed to the substrate 3. Numeral 6 designates a layer of hard solder. With this system, the bonding between the contact 1 and the substrate 3 is not improved. In addition, the contact 1 tends to become detached from the substrate 3 when the caulking of the mounting member 7 is loosened during the use of the contact.